1. Field of the Invention
The present invention pertains to determining the validity of seismic reflections and more particularly to determining the validity of seismic reflections that are interface reflections occurring below lateral velocity variations.
2. Related Prior Art
In acquiring seismic data, a seismic source is used to generate visco-elastic seismic acoustic waves that are directed into the earth's surface. At changes in the impedance of subsurface material, commonly called interfaces, the waves are partially reflected back in the direction of the seismic source at an angle governed by the law of reflection. The reflected portions of the waves are detected by seismic receivers. These receivers produce an electrical signal representing the reflected wave which may then be digitized and recorded. Further processing uses the recorded electrical signals to calculate the travel time of the wave, approximate the average velocity of the wave and determine the depth of the subsurface interface. In general, it is assumed that the recorded electrical signals represent (seismic) acoustic waves which have been reflected once.
The data obtained is usually presented in the form of time maps (e.g. seismic section, seismic trace, seismic sample, and the like) which are arranged according to time durations. The known factors are the source location, the receiver location and the time duration from generation to detection. The velocity of the seismic waves may be approximated by any one of the many methods presently in use. From these known quantities, the depth of the reflector is approximated by time migration followed by depth conversion. Depth conversion is the mapping of seismic traces on a depth scale, that is, converting data from time to depth. Time migration may be done-either before or after mid point or common depth point (CDP) data is stacked or combined.
Current seismic processing images p-wave reflection energy. For salt wedges, the reflection from a subsalt reflector is refracted, preventing the subsalt reflector from being imaged by time processing. Often, data that appears to be reflected data from beneath a salt overhang is mapped giving the appearance of accurate data showing various formations below the overhang. However, due to the nature of the overhang, this data could not be reflection data, at least not from the formations they inaccurately depict.
Much prior art has produced methods that attempt to image a subsurface formation below a significant lateral velocity differential, such as that of an irregular salt body. However, very little has been done to determine the reliability of time processed data that ostensibly depicts formations below the formations whose velocities vary laterally.
In general, the most commonly used prior art methods of time processing are incapable of determining the location of interfaces through surface generation of acoustic or seismic pulses when significant lateral velocity variations are present. Prior art methods of depth processing for accurately approximating these interfaces require very detailed velocity information and are not commonly used. In practice, most seismic data is processed using time processing. However, methods for determining the exact extent of reliable data from time processed data are not prevalent.